Thermoplastic rubber is a relatively new type of polymeric composition that has become commercially available within the past decade or so. These polymers have the useful property of behaving like rubber at normal use temperatures, without the need for vulcanization. And because they are not vulcanized, they can be processed by many of the normal thermoplastic resin procedures at elevated temperatures.
One serious problem has been encountered, however, in attempts to extrude film from thermoplastic rubber. Either because of too low melt strength, or excessive notch sensitivity, or both, pure thermoplastic rubber does not draw well when extruded into thin films. The preferred way to extrude films from thermoplastic polymers is to draw the extruded web at a rate of speed faster than the rate at which the molten material is ejected from the die. This causes a reduction in thickness. Typically, this reduction in thickness will be as much as three-fold or even more.
I have found, however, that when one attempts to extrude a film from pure thermoplastic rubber, the web cannot be drawn down at all. In fact, in many cases the web must be drawn at a rate of speed slower than the extrusion rate, to thereby produce a film having a thickness or gauge greater than the die gap. This is undesirable because the throughput rate is slow, and because gauge variations are magnified. And further, I have not been very successful in producing any films at all of pure thermoplastic rubber at thicknesses much less than about 10 mils because of an excessive tendency of the extruded webs to tear.
In my said earlier applications, I disclosed the addition of amorphous polypropylene to thermoplastic rubber to improve the processability of said rubber such that it can readily be extruded into films, while still retaining the characteristic properties of rubber. In application Ser. No. 67,850, I disclosed that thermoplastic rubber containing amorphous polypropylene can be directly extrusion formed into reticulated film in a continuous process which employs a pair of counter-rotating rolls, the first roll being a smooth roll and the second having a resilient surface engraved with a series of intersecting grooves.
It has now been discovered that, in addition to amorphous polypropylene, a number of other olefin polymers can be employed as additives to thermoplastic rubber in a continuous, direct extrusion process for the production of reticulated film having exceptionally valuable utility as the elastic member in elasticized disposable diapers.